Conventionally, oscillation circuits typically utilize an LC oscillator comprised of an inductor and a capacitor producing an inductance L and a capacitance C, respectively. The LC oscillator has a good stability and a relatively good S/N (signal to noise) ratio. However, the LC oscillator generally is unsuitable for an oscillation circuit intended to be fabricated on an IC (integrated circuit). This is because inductors are difficult to form on ICs.
Recently, new oscillation circuits, which are suitable for fabrication in ICs, were invented by one of the same inventors. The prior invention is now pending as Japanese Patent Application No. 61-172244 which was filed on July 22, 1986, and was filed in the U.S. on July 15, 1987 and assigned Ser. No. 07/074,294. The oscillation circuit according to the prior invention has been constructed by using band-pass filters. FIG. 1 shows an example of such a oscillation circuit using band-pass filters.
Referring now to FIG. 1, the oscillation circuit using band-pass filters according to the prior invention will be described. In FIG. 1, the oscillation circuit is comprised of a band-pass filter 10 and a limiter amplifier 12. An output terminal 10a of the band-pass filter 10 is coupled to its input terminal 10b through the limiter amplifier 12. Thus, the oscillation circuit forms a loop circuit consisting of the band-pass filter 10 and the limiter amplifier 12.
The band-pass filter 10 includes a pair of integration circuits 14 and 16. Each of the integration circuits 14 and 16 includes a voltage to current conversion circuit (referred as V/C converter hereafter) 18 and 20 respectively and a capacitor 22 and 24 respectively. Thus, each of the integration circuits 14 and 16 has a prescribed time constant defined by the conductance of the V/C converter and the capacitance of the capacitor. The V/C converters 18, 20 are constructed in a differential amplifier configuration.
The non-inverted input terminal 18a of the first V/C converter 18 is coupled to a DC voltage source 26 with a prescribed DC voltage. The output terminal 18b of the first V/C converter 18 is coupled to one end of the first capacitor 22 and the noninverted input terminal 20a of the second V/C converter 20. The output terminal 20b of the second V/C converter 20 is coupled to one end of the second capacitor 24 and the inverted input terminals 18c and 20c of the first and second V/C converters 18 and 20.
The inverted input terminal 18c of the first V/C converter 18 is directly coupled to the output terminal 20b of the second V/C converter 20. While the inverted input terminal 20c of the second V/C converter 20 is coupled to the output terminal 20b of the second V/C converter 20 through a voltage divider 28.
The output terminal 20b of the second V/C converter 20 is also coupled to the input terminal 12a of the limiter amplifier 12 through the output terminal 10a of the band-pass filter 10. The output terminal 12b of the limiter amplifier 12 is coupled to the other end of the first capacitor 22 through the input terminal 10b of the band-pass filter 10. By the way, the other end of the second capacitor 24 is grounded.
In the oscillation circuit, the output Va of the band-pass filter 10 on the output terminal 10a is fed back to the input terminal 10b through the limiter amplifier 12 in a positive phase relation. The transfer characteristics Tf of the band-pass filter 10 is expressed by the following equation: ##EQU1## where, gm18 is the conductance of the first V/C converter 18;
gm20 is the conductance of the second V/C converter 20; PA1 m is the voltage dividing ratio of the voltage divider 28 (m&lt;1); PA1 C22 is the capacitance of the first capacitor 22; and PA1 C24 is the capacitance of the second capacitor 24. Now, assuming that: ##EQU2## where .omega. represents an angular frequency, the Equation (1) is expressed as follows: ##EQU3##
When it is assumed that .omega.1=.omega.2=.omega.0, the Equation (3) becomes as follows: ##EQU4##
The transfer characteristic Tf of the band-pass filter 10, given by the Equation (4), has the frequency characteristics shown in FIG. 2, in which the graphs 2(A) and 2(B) show the frequency characteristics in regard to an absolute level La and a phase angle Ap of the transfer characteristics Tf.
When the output Va of the band-pass filter 10 having these characteristics is fed back to the input terminal 10b of the band-pass filter 10 in the positive phase relation through the limiter amplifier 12, oscillation takes place at the angular frequency .omega.0 as a resonance angular frequency. The resonance angular frequency .omega.0 varies in accordance with the conductances gm18 and gm20. Thus, the oscillation circuit can be operated as a voltage controlled oscillator by controlling the conductances gm18 and gm20.
In the oscillation circuit, as shown in FIG. 1, the input Vb fed back through the limiter amplifier 12 has been given by the voltage configuration. The limiter amplifier 12 generally includes an emitter follower type buffer (not shown) for supplying the input Vb of the voltage configuration. As is well known, emitter follower type buffers need a relatively large amount of current to operate. However, it is difficult to flow large currents in ICs, because ICs have become too large in scale for flowing a relatively large current.
If the emitter follower type buffer in the limiter amplifier 12 is driven by a relatively small current, the limiter amplifier 12 fails to supply the band-pass filter 10 with the input Vb in a stable state. Thus, the oscillation signal obtained by the oscillation circuit can be distorted. Further, an insufficient current for the emitter follower type buffer of the limiter amplifier 12 causes a phase delay in the oscillation signal. The phase delay may cause a shift of the oscillation frequency from a desired resonance frequency, i.e., the prescribed angular frequency .omega.0.
To avoid this problem, a push-pull type buffer may be considered for use in the band-pass filter 10 in place of the emitter follower type buffer. However, in this configuration the limiter amplifiers becomes more complicated occurs and more space is needed in the IC.
As described above in detail, in the oscillation circuit using band-pass filters according to the prior onvention, the oscillation frequency may shift from a desired resonance frequency if the band-pass filter is driven by a small current. On the other hand, a limiter amplifier for driving the band-pass filter becomes too large in scale and complicated in structure when a push-pull type buffer is used in the limiter amplifier for driving the band-pass filter.